Dielectric, pyroelectric, and ferroelectric studies in (1 - x)AgNbO3-xFeNbO4 lead-free ceramics.

In the present study, the effect of heterovalent Fe3+ ions on the dielectric, pyroelectric, and ferroelectric properties of the (1 - x)AgNbO3-xFeNbO4 (x = 0.005, 0.01, 0.025, 0.05, and 0.1) system was investigated. The substitution of smaller ionic radius Fe3+ in B-sites and the formation of FeNbO4 as a secondary phase contributed to improved dielectric performance, especially the pyroelectric effect, of (1 - x)AgNbO3-xFeNbO4 ceramics by generating electron-rich ceramics. The (1 - x)AgNbO3-xFeNbO4 ceramics were prepared by conventional solid-state sintering. Pure AgNbO3 had a perovskite crystal structure with an orthorhombic crystal system, but the FeNbO4 in (1 - x)AgNbO3-xFeNbO4 ceramics was formed as a secondary phase with a monoclinic structure. In addition, the XRD and Raman spectroscopy data showed that some Fe3+ was substituted into B-sites of AgNbO3. The introduction of FeNbO4 effectively reduced the average grain size from 1.85 ± 0.09 µm to 1.22 ± 0.03 µm for pure AgNbO3 and 0.9AgNbO3-0.1FeNbO4, respectively. In addition, the relative density of the (1 - x)AgNbO3-xFeNbO4 ceramics decreased from 97.96% ± 0.01 for x = 0 to 96.75% ± 0.03 for x = 0.1. The real part of the permittivity ε', at room temperature, increased from 186.6 for x = 0 to a value of 738.7 for x = 0.1. Additionally, the maximum pyroelectric coefficient increased fivefold, reaching values of 2270 nC cm-2 K-1 for x = 0.1. Furthermore, a harvested pyroelectric energy density (W) of 1140 µJ cm-3 for x = 0.025 was achieved, which is appreciably higher than the 840 µJ cm-3 value for x = 0.

Improvement of CO Oxidation and CH4 Combustion by Pd and Pt Partial Substitution on LaMn0.5Cu0.5O3 Perovskite.

LaMn0.5Cu0.5O3 (LMC) as the parent perovskite and Pd- and Pt-doped LaMn0.5Cu0.5O3 catalysts (LMCPd and LMCPt) instead of Cu were synthesized in a new solid-state synthesis technique at a low temperature. Perovskite lattice formation of the LMC catalyst was successfully performed at 600 °C. All perovskites were investigated by X-ray diffraction, HRTEM, O2-TPD, H2-TPR, BET, and XPS analyses. The prepared perovskites were used as heterogeneous catalysts for CO oxidation and methane combustion reactions. The catalytic performance of the LMC catalyst was noticeably enhanced via Pd and Pt substitution instead of Cu. The enhancement in the mobility of lattice oxygen and specific surface area has triggered this catalytic performance improvement, which play an important role in CO oxidation and methane combustion. The Mn 2p and Mn 3s XPS spectra showed that by doping Pd and Pt in the LMC perovskite, Mn was affected in different states and the Mn 3s peaks were only observed in the LMCPt catalyst. XPS spectra of the LMCPd1 sample showed a high oxidation state of Pd3+ or Pd4+, from which it can be concluded that Pd was successfully incorporated into the LMC perovskite lattice. The H2-TPR profiles of the LMCPd and LMCPt perovskites revealed that the reduction peaks of Cu and Mn were shifted to lower temperatures by increasing Pd and Pt partial substitution due to the synergetic effect of the cation and the H2-spillover effect of palladium and platinum.

Enhanced energy storage performance in reaction-sintered AgNbO3 antiferroelectric ceramics.

In this research, AgNbO3 ceramics were produced by two sintering methods: reaction sintering (RS) and conventional solid-state sintering (CSSS). The process was similar for both methods, except that in RS, Ag2O and Nb2O5 precursors were mixed, then formed into pellets, skipping the calcination step, and sintered at 1100 °C for 6 hours. Both prepared ceramics had the same perovskite crystal structure with an orthorhombic crystal system and Pbcm and Pmc21 space groups with similar lattice dynamic vibration modes at room temperature. The average grain size of the polycrystalline samples prepared by RS and CSSS was found to be ∼2.03 ± 0.77 and ∼1.85 ± 0.96 µm, respectively. The relative bulk densities of the ceramics produced by RS and CSSS were found to be ∼94.0 ± 1.8 and ∼96.5 ± 1.3%, respectively. Ceramics prepared by both methods showed antiferroelectric behavior, and reaction-sintered AgNbO3 ceramics exhibited lower energy loss density than CSSS samples. In addition, a recoverable energy storage density (Wrec) of 3.1 J cm-3 and higher energy storage efficiency (η) for RS samples were measured at 175 kV cm-1. Moreover, the η values of 74.2% and 57.7% were measured for samples sintered by RS and CSSS, respectively. This energy storage efficiency is the highest ever reported for pure AgNbO3 ceramics. Furthermore, reaction-sintered samples showed good temperature stability for Wrec and η in the 30-80 °C temperature range.

Effect of in situ VSi2 and SiC phases on the sintering behavior and the mechanical properties of HfB2-based composites.

In situ HfB2-SiC-VSi2 composite was fabricated by reactive pressureless sintering at the temperature of 2150 °C for 4 h under a vacuum atmosphere. In situ SiC and VSi2 reinforcements were formed using VC and Si powders as starting materials according to the following reaction: VC + 3Si = SiC + VSi2. Microstructural studies and thermodynamic calculations revealed that in situ VSi2 and SiC phases were mostly formed and homogeneously distributed in HfB2 skeleton. The results showed that the density of in situ HfB2-SiC-VSi2 composite was 98%. Besides, the mechanical properties of the composite were effectively enhanced by the formation of in situ second phases. The Vickers hardness and the fracture toughness of the composite reached 20.1 GPa and 5.8 MPa m-1/2, respectively.

Synthesis of nano- alumina powder from impure kaolin and its application for arsenite removal from aqueous solutions.

Adsorption is considered a cost-effective procedure, safer to handle with high removal efficiency. Activated alumina is the most commonly used adsorbent for the removal of arsenic from aqueous solutions. However, activated alumina has a low adsorption capacity and acts kinetically in a slow manner. An ideal adsorbent should have a high surface area, physical and/or chemical stability and be inexpensive. To meet this requirement, nanomeso porous γ-alumina with a high surface area (201.53 m2/g) and small particle size (22-36 nm) was prepared from inexpensive kaolin as the raw material, by precipitation method. The research results showed that adsorbent has the high adsorption capacity (for initial arsenite concentration up to 10 mg/L, in which 97.65% recovery was achieved). Optimal experimental conditions including pH, initial arsenite concentration and contact time were determined. Langmuir, Freundlich and Dubinin- Radushkevich isotherm models were applied to analyze the experimental data. The best interpretation for the experimental data was given by Langmuir adsorption isotherm equation and the maximum arsenite adsorbed by synthesized nano γ-alumina (qe) was found to be 40 (mg/g).